Energy management is one of the most challenging issues in the world today. Accordingly, various energy harvesting technologies have gained attention, including harvesting energy from walking or other vibration sources using piezoelectric materials. Self-powered supply sources are an emergent need for, among other things, communication devices; sensor networks for infrastructures; health monitoring and active control for aircraft; and soldiers on the battleground. Traditionally, there are three kinds of energy resources: heat, solar, and motion/vibration. Among them, heat and solar energies are weather dependent, which cannot be controlled by our soldiers for harvesting energy in military applications. The only energy resource that can be controlled by our army and marine soldiers is motion/vibration energy. In addition, ocean surface waves are a huge motion energy resource for naval battlefields. Vibrations from engines or other motion structures in any kind of battlefield are the other motion energy resources to be harvested. A piezoelectric structure is one of the common devices used to harvest motion/vibration energy. For aeronautic and space missions, piezoelectric energy harvesting transducers can harvest energy for auxiliary energy and reduce noise in aircraft. Historically, most researchers focused on cantilever based energy harvesting devices, which work at a resonance mode to harvest motion/vibration. See, e.g., S. Beeby, M. Tudor, and N. White, “Review Article: Energy harvesting vibration sources for microsystems applications,” Measurement Science and Technology, 17 R175-R195, 2006; and S. Anton and H. Sodano, “Topical Review: A review of power harvesting using piezoelectric materials (2003-2006).” Smart Materials and Structure 16 R1-R21, 2007. Some researchers demonstrated that a flextensional (Moonie) transducer offers more efficiency to harvest energy than a cantilever based device. See, e.g., H. Kim, S. Priya, H. Stephanou, and K. Uchino, “Consideration of Impedance Matching Techniques for Efficient Piezoelectric Energy Harvesting,” IEEE Tran, UFFC 54(9), 2007. A broadband, high efficiency motion/vibration energy harvesting device to harvest human motion energy, wind energy, ocean surface wave, as well as environmental vibration energy that can tap such unconventional but viable energy sources remains a goal of many scientists.
Review of conventional piezoelectric energy harvesting transducer (PEHT) structures, revealed that most effective piezoelectric constants are lower than about 104 pC/N, (resonant mode). This is, obviously, the main reason why such conventional PEHTs cannot harvest enough electric power. Recent technology developments have focused on hybrid piezoelectric stack materials that include layered electromechanical improvements in order to increase conversion efficiency in promising ways, such as, for example, U.S. Pat. No. 7,446,459 and U.S. Patent Application Pub. No. 2010/0096949, which are both incorporated herein by reference thereto in their entirety. However, continued improvement is sought in other parts of PEHT structures, including mechanisms for directing and amplifying forces into such stack materials.
Further, for a specific vibration/motion resource, there needs to be a means to couple more mechanical energy from the vibration structure into a piezoelectric device and a means to convert a greater fraction of such mechanical energy into electrical energy. First, in order to couple the mechanical energy into PEHTs more effectively, the PEHT structure needs to be optimized and defined by the natural and environment of the vibration/motion resource. The performance of the PEHT device design depends on the piezoelectric properties and the form of the piezoelectric material. Piezoelectric materials with higher electromechanical coupling factors and piezoelectric constants may be preferred. Moreover, piezoelectric multilayers may improve reliability, durability, energy storage capability and integration capability.
In addition, many civilian and military applications require high performance electromechanical actuators. These include vibration control, dynamic flow control in aerospace, underwater navigation and surveillance, microphones, etc. High displacement and high electromechanical output power are two main demands for actuators needed in many applications. Historically, a great deal of effort has been devoted to two research fields: 1) the development of electromechanical active materials offering the desired properties and 2) the development of electromechanical devices which utilize the materials in an efficient manner. Since the development of metal ceramic actuators (called “Moonies”), many device configurations have been exploited for amplified displacement and enhanced efficiency.
In order to improve mechanical energy output, hybrid actuation system (HYBAS) may be used to utilize the characteristics of the electromechanical performance of these two types of electroactive materials in cooperative and effective way. Such a system shows significantly-enhanced electromechanical performance compared to the performance of devices made of each constituent material individually. A theoretical model for the HYBAS considers the elastic and electromechanical properties of the materials utilized in the system and the device configuration. Other actuator technologies (synthetic jet) include piezoelectric hybrid energy harvesting transducer (HYBERT), piezo triple hybrid actuation system (TriHYBAS), and piezoelectric multilayer-stacked hybrid actuation/transduction systems (Stacked-HYBATS) based on the understanding of the electromechanical properties of piezoelectric materials and their applications.